Range extension of invasive Cancer irroratus and native Carcinus maenas polewards in the Ascophyllum-dominated intertidal zone in north-west Iceland

The rocky intertidal zone of sheltered shores in Breiðafjörður, north-west Iceland is dominated by monospecific stands of canopy-forming brown algae Ascophyllum nodosum, which provide habitat for mobile organisms and has been subjected to long-standing rotational harvesting. We investigated the assemblage composition of little-studied mobile brachyuran crabs in this area, to track distributional shifts in a native species responding to climate change and extent of occupancy of the intertidal by a primarily subtidal invasive non-native species. Potential interactive effects of seaweed harvesting were explored. Breiðafjörður was compared with two reference sites in Faxaflói, south-west Iceland. The study revealed clear poleward expansion of the native European green crab Carcinus maenas in the region, displacing the native spider crab Hyas araneus particularly at mid-shore levels. The invasive non-native Atlantic rock crab Cancer irroratus had negligible occupancy in the intertidal zone, indicating limited effects on the intertidal crab assemblage, composition, and abundance. The current harvesting regime of A. nodosum in Breiðafjörður did not affect the composition and abundance of the brachyuran crab assemblage in the rocky intertidal zone. H. araneus is likely being squeezed by displacement subtidally by C. irroratus, and intertidally by C. maenas. Overall, we provide insights into the potential interactions between climate change, invasive species, and human activities in the rocky intertidal zone.